Traveling Waves are continuous in amplitude, in space and time. A transducer located in space samples every wave passing it, producing a composite continuous amplitude function of continuous time, thereby “capturing” the whole wave. In addition to a transducer or antenna, field deployed sensors typically include signal processing capability.
In order to measure phase, a reference is required. The reference may be a reference signal, as occurs in phase detectors, or time as used in Global Positioning Systems. The reference may be located at each sensor or at a central processing site. If the reference is deployed in the field, all the sensors use of the references must be synchronized. Typically, this would be accomplished by transmitting synchronization from a central site to the field. Later, the sensed signals are shipped from the field to the central site. Often to save synchronization cost, the whole wave is shipped from multiple field sensors to a central site, where the sensed signal is compared to a reference signal, eliminating the need for synchronization in the field. The price of shipping the whole wave from the field is usually the need for wide bandwidth transmission. Sensed signals may be shipped in analog or digitized form prior to being sent back.
In most prior art signal processing, computation aggregates all or parts of the signal over several wave cycles by, for example, frequency shifting, phase shifting and summing, heterodyning, time-domain low pass filtering, integration, cumulation, box car integrating, running mean, time sequence smoothing, trend line/curve fitting, or averaging of the signal. Aggregating the signal for several cycles requires the group of sensors remain relatively stationary during the cycles being combined.
Time Delay of Arrival (TDOA) is a generalization of radar ranging. Typically, a modulation structure is introduced by, for example, transmission of a pulse. By comparing the times of arrival of the modulation structure at multiple receiving sensors, the direction of the wave can be computed.
One important area where prior art has not engineered practical solutions is in providing flexible, low cost, light weight, directional reception of low frequency, long wave length signals. The use of antennae or transducers to determine wave vector direction is limited by the directionality of the transducer. The directionality of the transducer is limited by the size of the transducer compared to the longest wavelength of the wave being transduced. This leads to practical limitations in the case of low frequencies with correspondingly long wavelengths. Fundamental and practical limits arising from the physics and mathematics of waves dictate that significant directionality is only achieved when size of the entire antenna structure is significantly larger than the wavelength being received.
That the wavelength sets the natural geometric scale for the antenna or transducer array is discussed in Radar System Engineering, MIT Radiation Laboratory Series Volume 1 section 9-1 on page 271: “ . . . . Another advantage of large antennas, having to do with the resolution of the radar set, is that the beamwidth varies inversely as the linear dimension of the antenna. Mathematically, the beamwidth Θ (degrees) is usually related to the width D of the antenna and the wavelength λ of the radiation by the approximate formula Θ≈70λ/D if D and lambda are in the same units.” This relationship provides a rule of thumb for estimating the directionality that can be achieved when the size of an antenna system is known, or to estimate the size of antenna system needed if directionality is specified.
Freely propagating waves are used for many purposes, such as radio and TV broadcasts, microwave communications links, RADAR, SONAR, geophysical prospecting, earthquake mapping, ultrasonic imaging, satellite communications, cell phones and wireless networking.